[PATCHSET RFC 0/4] Support passing fds between chain links

[Jens Axboe <axboe@kernel.dk>]

One of the fabled features with chains has long been the desire to
support things like:

<open fileX><read from fileX><close fileX>

in a single chain. This currently doesn't work, since the read/close
depends on what file descriptor we get on open.

This is very much a RFC patchset, but it allows the read/close above
to set their fd to a magic value, IOSQE_FD_LAST_OPEN. If set to this
value, the file descriptor will be inherited from the last open in
that chain. If there are no opens in the chain, the IO is simply
errored. Only a single magic fd value is supported, so if the chain
needs to operate on two of them, care needs to be taken to ensure
things are correct. Consider for example the desire to open fileX
and read from it, and write that to another file. You could do that
ala:

<open fileX><read from fileX><close fileX><open fileY><write to fileY>
	<close fileY>

and have that work, but you cannot open both files first, then read/write
and then close. I don't think that necessarily poses a problem, and
I'd rather not get into fd nesting and things like that. Open to input
here, of course.

Another concern here is that we currently error linked IO if it doesn't
match what was asked for, a prime example being short reads. For a
basic chain of open/read/close, the close doesn't really care if the read
is short or not. It's only if we have further links in the chain that
depend on the read length that this is a problem.

Anyway, with this, prep handlers can't look at ->file as it may not be
valid yet. Only close and read/write do that, from a quick glance, and
there are two prep patches to split that a bit (2 and 3). Patch 1 is just
a basic prep patch as well, patch 4 is the functional part.

I added a small 'orc' (open-read-close) test program in the fd-pass
branch of liburing:

https://git.kernel.dk/cgit/liburing/plain/test/orc.c?h=fd-pass

as an example use case.

-- 
Jens Axboe